This invention relates to injection molded plastic compositions produced by an extrusion process from a mixture of raw materials including cellulose acetate, whole starch granules and plasticizers. The invention also relates to injection molded plastics containing a range of intact starch granules and their method of manufacture which includes making plastic pellets or molding parts directly from blended powders extruded using both single and twin screw equipment.
Cellulose acetate has been commercially produced for more than 60 years. Different raw materials have been used such as cotton, recycled paper and wood cellulose. The cellulose acetates produced have mainly been in the form of white powders ranging from di to tri acetates mostly for plastic sheets and molded parts. Plasticizers used in the prior art have included triacetin which is made from a reaction with glycerol and acetic acid and other reagents. Most commercial cellulose acetate products are clear, strong, stiff and have a broad range of applications. Cellulose is usually reacted with acetic anhydride. However, for more expensive high performance plastics, other anhydrides may be used such as butyric and proprionic.
Recent emphasis on biodegradable plastics has led to investigations on the biodegradable properties of cellulose acetates. Refer to "Biodegradation of Cellulose Acetates", Charles M. Buchanan et al, Eastman Chemical Company, 1992. In general, those cellulose acetates which have an acetyl substitution number of 2.2 or less are biodegradable in soil and marine environments as well as being compostable. Higher substitution numbers ranging from 2.2 to 3.0 are less biodegradable. Other factors in the rate of biodegradation include the type and quantity of plasticizer present and the amount of residual sulfuric and hydrochloric groups which are introduced in the initial reaction in the form of sulfuric acid or hydrochloric acid or blends of the two, as a homogeneous catalyst. Typical reaction compositions to produce cellulose acetate using cotton fibers are: 2:1 glacial acetic acid to cotton, 1.5:1 acetic anhydride to cotton and 65:1 cotton to sulfuric acid. Reaction temperatures usually are maintained between 5.degree. C. and 35.degree. C.
For cellulose acetate syntheses refer to:
1. U.S. Pat. No. 1,986,908, "Production Of New Compositions Of Matter By The Esterification of Cellulosic Material", Victor Emmanuel Yarsley, Jan. 8, 1935. PA0 2. U.S. Pat. No. 2,040,801, "Method For Production Of Cellulosic Materials From Waste Cellulose Products", Wyly M. Billing, May 12, 1936. PA0 4. U.S. Pat. No. 5,205,863 "Agricultural Biodegradable Plastics", Glenn R. Elion, Apr. 27, 1993. PA0 5. U.S. Pat. No. 4,673,438, "Polymer Composition For Injection Molding", Fritz Wittwer, Jun. 16, 1987. PA0 6. U.S. Pat. Application Ser. No. 08/010,735, "Method of Producing Biodegradable Starch-Based Product From Unprocessed Raw Materials", Jean M. Mayer et al, Jan. 29, 1993. PA0 7. U.S. Pat. Application Ser. No. 08/047662, "Starch Based Biodegradable Blown Film Compositions And Methods Of Manufacture", Jean M. Mayer et al, Apr. 13, 1993.
3. U.S. Pat. No. 4,016,353, "Process For Producing Organic Acid Esters Of Cellulose", David Goheen et al, Apr. 5, 1977.
Starches have been used in injection molded plastics and in blown films. In some cases the material composition can be 100% starch, and in other cases the starch is mixed with water, glycerin and other compounds including vinyl alcohols. For related prior art references see:
In the prior art, the material costs for injection molded plastics using starch blends were too high to be competitive with oil and gas derived plastics. This is because the starch must be reacted and formed under high temperature and pressure into gelatinized or destructured starch pellets and then compounded with other copolymers and additives for various applications. Further, in the prior art, biodegradable plastics containing starch have the intrinsic problem of being too reactive with water to make stable products and most starch products, especially destructurized starch, embrittle over time or at elevated temperatures, rendering them useless in many cases or at the least dramatically reducing their tensile strength and other physical properties relating to structural integrity, such as flexibility, percent elongation at breaking, and toughness.
In the prior art, the most common plasticizer used with cellulose acetate was triacetin. This combination can produce strong clear plastics which are slightly biodegradable but too expensive for mass production and too expensive to compete with oil and gas derived plastics for many applications using plastics such as polystyrene and low or high density polyethylene.
A new composition of matter and method of manufacturing is needed to make cellulose acetate based plastics easily biodegradable, strong, yet of low cost. This invention described below achieves these goals.